In the field of high-speed printing devices which are especially suitable for use in connection with electronic business systems, solenoid driving circuits receive extensive use. These driving circuits are used for activating printer hammers and the solenoids of wire matrix type printers. Such circuits require rapid cycling times because the cycle times of these circuits are a direct function of the speed at which printing may be accomplished.
A prior art patent of interest for setting out the state of the art is U.S. Pat. No. 3,089,960, entitled "Electrical Drive Circuit", by G. G. Bailey. The drive circuit of the aforementioned patent divides the primary winding of a transformer into at least two portions which are inductively coupled to the secondary winding. Activation of a load device that is connected to the secondary winding, reflects a signal from the secondary winding back to the primary winding. The reflected signal lowers the potential established at a particular point in the drive circuit to allow a circuit conductive device to effectively establish a new turns ratio with respect to the first and second portions of the primary winding. The new turns ratio operates to induce a lower voltage in the secondary winding, which voltage is still of a sufficient magnitude to hold the load device in its activated condition. Generally then, the aforementioned circuit operates to automatically alter the turns ratio of the drive transformer so as to establish distinct predetermined voltages at the output. These predetermined voltages are of a sufficient magnitude to activate the load device, and to maintain the load device in an activated condition without the constant use of high power and/or high peak currents.
Another circuit of interest is disclosed in U.S. Pat. No. 3,560,821, entitled "Pulse Type Drive Circuit for an Inductive Load", by T. E. Beling. The circuit of the aforementioned patent utilizes a polarizing circuit for returning current from an inductive load back to the source in order to attempt to minimize the fall time associated with the changes in driver current. One of the desired side effects of the subject invention is to greatly decrease the power requirements of the circuit while maintaining a high degree of efficiency.
Another patent of interest is U.S. Pat. No. 3,628,102, entitled "Exciter Apparatus for Impact Member Solenoid", by K. E. Jauch. As part of the driving circuit of the referenced patent, there is provided a protective energy-absorbing network that consists of a resistor and a diode which are connected across the terminals of a solenoid. The resistor and diode combination provide a current path for the inductive, or fly-back current, flowing in the solenoid upon removal of power. As the voltage across the terminals of the solenoid reverse, the energy stored in the inductance of the solenoid attempts to maintain the current flowing in the solenoid, which, in turn, causes the inductor to act as a power supply. The resistor is utilized to dissipate this power. The use of a resistance to dissipate power through heating is an inefficient use of energy; in addition, the resistance also causes a delay in the rise and fall time of the driving voltages associated with the driving of the inductive load.
Four additional patents of interest for showing the state of the art of the inductive driver circuits are: U.S. Pat. No. 3,078,393, entitled "Driver for Inductive Load", by C. R. Winston; U.S. Pat. No. 3,411,045, entitled "Electrical Circuits for Rapidly Driving an Inductive Load", by A. D. Baker et al.; U.S. Pat. No. 3,705,333, entitled "Adjustable Active Clamp Circuit for High-Speed Solenoid Operation", by L. V. Galetto et al.; and finally, U.S. Pat. No. 3,859,572, entitled "Magnetic Coil Driver Circuit", by S. D. Kidl et al.
None of the aforementioned patents attempt to decrease the turn-on and turn-off of an inductance driver by utilizing the properties of a tertiary transformer.